BULLETIN DE L ’INSTITUT ROYAL DES SCIENCES NATURELLES DE BELGIQUE SCIENCES DE LA TERRE, 74-SUPPL.: 205-212, 2004BULLETIN VAN HET KONINKLIJK BELGISCH INSTITUUT VOOR NATUURW ETENSCHAPPEN AARDW ETENSCHAPPEN, 74-SUPPL.: 205-212, 2004

Ecology and evolution o f Pliocene bivalves from the Antwerp Basin

by Robert M A R Q U ET

M a r q u e t , R., 2005. - Ecology and evolution o f Pliocene bivalves from the Antwerp Basin. Bulletin de I ’Institut royal des Sciences naturelles de Belgique, Sciences de la Terre, Sciences de la Terre 74 supplement: 205-212, 5 figs, 3 tables; Bruxelles-Brussel, December 15, 2004. - ISSN 0374-6291

Abstract

The dock construction works in the port o f Antwerp at Doei and Kallo (prov. Oost-Vlaanderen, Belgium) have allowed the collection o f a rich and diverse Pliocene bivalve fauna. It contains 182 taxa, 178 species and four subspecies. 90 species out o f 178 (51%) are extinct. The rate o f extinction does not significantly vary between the recognised strata, ranging from Lower to Upper Pliocene. The average longevity o f the species is calculated at 10.7 Ma. This long range makes most o f these bivalve species unfit for use in Neogene stratigraphy. The bathymetry o f the different levels o f the Pliocene is assessed, using the species still occurring in the Recent fauna (49% o f the total number): it decreases towards the end o f the Pliocene.

Key words; Stratigraphie range - Bathymetry - Pliocene - Bivalves.

Résumé

Les travaux de construction des docks dans le port d ’Anvers à Doei et à Kallo (prov. Oost-Vlaanderen, Belgique) ont permis de récolter une faune de bivalves pliocènes riche et diverse. Elle contient 182 taxa, 178 espèces et quatre sous-espèces dont 90 espèces (51%) sont éteintes. Le pourcentage d ’extinction ne varie pas de façon significative entre les différents niveaux reconnus, qui datent du Pliocène inférieur au Pliocène supérieur. La longévité moyenne des espèces est de 10,7 millions d ’années. Cette longue distribution dans le temps rend impossible d’utiliser la plupart des espèces de bivalves pour la biostratigraphie du Néogène. La bathymétrie des niveaux pliocènes est calculée, en utilisant les taxa encore présents dans la faune Recente (49% du nombre total): elle diminue vers la fin du Pliocène.

Mots-clefs: Durée stratigraphique - Bathymétrie - Pliocène - Bivalves.

Introduction

The dock construction works in the Antwerp port area at Kallo and Doei (Oost-Vlaanderen, Belgium) started in 1970 and still continue today. During the construction several sections were exposed, ranging in age from the Oligocene to Late Pleistocene. The Pliocene in the area discussed contains two formations with five members: the Kattendijk Fm. (Lower Pliocene) with the Kattendijk

Sand Member and the Lillo Fm. (Middle and Upper Pliocene) with the Luchtbal, Oorderen, Kruisschans and Merksem Sand Members. Within the Oorderen Sand Member, several levels can be recognised: a basal crag and the Atrina, Cultellus and Angulus benedeni levels (see Table 1). In general, the Pliocene yielded an extensive, well-preserved and diverse mollusc fauna. The only exception is the Merksem Sand Member, which contains a very poor and partially derived association. In this paper the Bivalvia from the different Pliocene levels will be considered, but not those from the basal crag o f the Oorderen Sand Member, because it contains a large number of derived specimens.

Table 1 — Stratigraphy of the Pliocene in the Antwerp Basin, and correlation with the international stratigraphie scale; modified after V a n d e n b e r g h e et al., 1998. I.S.U. = International Stratigraphie Units, BM = benthic molluscs, BF = benthic Foraminifera, PF = planktonic Foraminifera, O = otoliths.

I.S.U. Form ations M em bers Levels BM BF PF 0

Zandvliet

Gelasian Merksem BM 22C

Kruisschans

LilloAngulusbenedeni B 12

19

Piacenzian Oorderen Cultellus BM 22B

Atrina

basal crag

ZancleanLuchtbal BM 22A B l l 18

Kattendijk Kattendijk

Glycymeris-Ditrupa

BM 21C B IONPF

17Petaloconchus

16

basal bed

206 Robert MARQUET

Stratigraphie range o f the bivalve species

The Pliocene Bivalvia from Doei and Kallo are currently being taxonomically revised by MaRQUET (2002, 2004 in press). It consists of 178 nominal species and four subspecies. O f these species 90 (51%) are extinct, whereas 88 (49%) still occur in the Recent fauna (Table 2). Only the latter will be used for the palaeoecological analysis. Although the number o f species decreases after the Lower and Middle Pliocene (Kattendijk and Luchtbal Sand Members respectively), the rate o f extinction remains more or less the same. This implies that the extinction o f Pliocene species occurred at the onset o f or during the Pleistocene.

The average longevity of the Bivalvia found at Doei and Kallo is calculated at 10.7 Ma (Fig. 1). This however is an underestimation, because many species still occur and their life span will be longer when they become extinct. A large stratigraphie range consequently may be postulated for most Cenozoic Bivalve species. Nearly 100 species out o f 178 have a range of less than 10 Ma, but the remaining species mostly lived between 10 and 20 Ma. Some even ranged from the Upper Eocene to the Recent (e. g. Corbula gibba ( O l i v i , 1792)). Because of their long stratigraphie range, the aforementioned species cannot be used in a more detailed biostratigraphy o f the Neogene. Only the longevity o f species (including all subspecies) is considered here. However, it should be noted that the range calculated in this way is taxonomically biased: much depends on the status (subspecies or separate species) that has been accorded to taxa. This for example is also the case for Corbula gibba, which according to G l i b e r t & V a n D e P o e l (1966) consists of four subspecies, two o f which have been interpreted as full species by other authors.

M a r q u e t (1996, 1997, 1998, 2001) revised the taxonomy of a number o f gastropod families occurring in the Pliocene of Kallo and the Antwerp region in general: Conidae-Turridae, Cerithiopsidae and Triphoridae. A large number o f these gastropod species showed, in con-

120

u 100

60

60

40

20

00-10 20.1-30 30.1-4010.1-20

Fig. 1 — Longevity of the bivalve species found in the Doei and Kallo sections.

trast to the Bivalvia considered herein, a very short time range, remaining limited to respectively the Lower, Middle or Upper Pliocene. They could be useful in biostratigraphy, but only in a limited geographical area, since they are mainly endemic to the North Sea Basin. It could be concluded that the species turnover is larger and the longevity o f the species shorter in gastropods than in bivalves. However, also here a taxonomic bias could be present. Indeed, among the Gastropoda the protoconch is a very fine tool for species identification. It is assumed that all populations differing in number o f protoconch whorls and/or protoconch ornament belong to different species. For Bivalvia, a comparable character that fossilizes and allows identification lacks. The hinge is characteristic for species groups at the generic level, family or even higher, but not for individual species.

Bathymetry o f the Kallo and Doei Pliocene deposits

Only the extant species will be considered herein, because accurate data about their ecology are available. We do not accept a priori that extinct fossil species have the same ecological requirements as extant congeners. Overlap o f ecological habitat and niche would result in heavy interspecific competition, quickly eliminating one o f the competing species. However, in the Pliocene fauna of Antwerp it is proven that most species have a long stratigraphie range. In the living fauna too, species from the same genus always show differences in one or several ecological parameters. A disadvantage o f this restriction is, that short living species are excluded from the analysis. In the Pliocene fauna of the North Sea Basin this is especially important for the family Astartidae, well represented in species as well as in individuals, but these species mostly have a short stratigraphie range, not reaching to the present.

The water depth ranges o f the Recent bivalve species found in the Pliocene sections at Kallo and Doei were taken from the compilation work o f P o p p e & G o t o (1993), but exceptional occurrences were excluded (see Table 2). Thus only 75 out o f the 88 extant species could be taken into consideration. These are listed in table 3 in M a r q u e t (2004, in press). For each species, the distribution was recorded for the Kattendijk Fm. and the Luchtbal, Oorderen and Kruisschans Sand Members o f the Lillo Fm., but not from the Merksem Sand Member, because the data available for this level were too poor. Two different approaches were used:- In a histogram, the total number of species occurring at

each bathymetric interval was counted. Intervals of 10 m were used below 100 m and intervals o f 100 m between 100 and 1000 m. This yielded average depth values for each level, for example 50 to 70 m for the Kattendijk Sand Mbr. (see Figs 2 to 5).

- The minimum and maximum bathymetric occurrences o f the species for each level were noted and the common overlap area o f these species was delimited. This

Pliocene bivalves North Sea Basin 207

Table 2 — Bivalve species found in the Kallo and Doei sections. Bat = Bathymetry; e = early; m = middle; 1 = late; Eo = Eocene;01 = Oligocene; Mi = Miocene; Pli = Pliocene; Pie = Pleistocene; R = Recent; K = Kattendijk; L = Luchtbal; OA = Oorderen Atrina; O C= Oorderen Cultellus; OAb = Oorderen Angulus benedeni; Ks = Kruisschans; M = Merksem

SPECIES RANGE BAT. K L OAOC

0Ab

KS

M

Nucula (N.) nucleus eMi-R 10-400 ■ ■ ■ ■ _ ■ _Nucula (N.) trigonula eOl-mPli - ■ ■ ■ ■ — ■ -Nucula (Lamellinucula) cf. jeffreysi eMi-ePle - ■ - - - - - -Leionucula 1. laevigata uOl-lPli - ■ - ■ ■ ■ ■ -Acula (Truncacila) cobboldiae lPli-Ple - - - - - - 7Yoldiella philippiana wesselinghi eMi-R ? ■ - - - - - -Yoldia (Y.) semistriata e-mPli - ■ ■ ■ ■ - - -Yoldia (Y.) heeringi lPli - - - - - ■ -Arca (A.) tetragona eMi-R 0-2600 ■ ■ - - - - -Barbatia (B.) barbata eMi-R 0-280 ■ - - - - - -Barbatia (B.) philippiana ePl-R 150 ■ - - - - - -Bathyarca pectunculoides eMi-R 40-1000 ■ ■ - - - - -Stiarca scaldensis eMi-ePli — ■ - - - - — -Limopsis (L.) aurita 101-R 100-900 ■ - - - - -Limopsis (Pectunculina) anomala coxi IMi-lPli - ■ ■ - - - - -Nucinella ovalis eMi-lPli - ■ ■ - - — ■ —Glycymeris (Chevronia) variabilis ePli-lPli - ■ - - ■ ■ -Glycymeris (Chevronia) obovata ringelei Glycymeris (G.) radiolyrata pseudodeshayesi

eOl-ePli — ■ ■■

— — — — —

Glycymeris (G.) r. radio lyrata ePli-mPl - - ■ ■ ■ — —Mytilus edulis ePli-R 0-40 ■ - ■ ■ ■ ■ —Crenella decussata IMi-R 4-200 ■ — - - — — —Arcoperna sericea IMi-ePle - ■ ■ ■ ■ - - -Gregariella semigranata eMi-R 2-100 ■ ■ - - - - -Musculus discors ePli-R ? ■ — - - - — -Modiolarca subpictus ePli-R 0-60 ■ ■ - - - -Rhomboidella pridauxi eMi-R ? ■ ■ - - - - -Rhomboidella grigisi eMi-ePli - ■ - - — — — —Modiolula phaseolina eMi-R 0-160 ■ ■ ■ - ■ - -Modiolus modiolus IMi-R 0-150 ■ - — - — —Atrina fragilis kai loens is ePli-R? 150-600 ■ ■ ■ ■ ■ ■ -Pteria phalaenacea eMi-ePli ? ■ - - - - -Pecten grandis IMi-mPli - ■ ■ - - - - -Pecten complanatus m-lPli - - - ■ ■ ■ -Pseudamussium princeps IMi-ePli - ■ - - - - - -Pseudamussium lilli eMi-ePli - ■ ■ — — — — —Pseudamussium sulcatum mMi-R — ■ — — — — — —Pseudamussium clavatum IMi-R 0-2500 ■ — — — — — —Palliolum tigerinum eMio-R 5-1400 ■ ■ ■ ■ - — —Palliolum gerardi IMi-ePli 0-400 ■ ■ — — — — —Delectopecten vitreus ePli-R - ■ - - - - - -Similipecten similis eMi-R 30-600 ■ ■ - ■ - - -Aequipecten o. opercularis eMi-R 4-250 ■ ■ ■ ■ ■ ■ ■Aequipecten radians eMi-mPli 0-400 ■ ■ - - - - -Aequipecten wagenaari e-mPli - ■ ■ ■ ■ - -Crassodoma m. multistriata Crassodoma multistriata harmeri eMi-R

— ■ ■■ ■

— — —

Monia patelliformis mPli-R 10-180 ■ ■ ■ ■ ■ —Heteranomia squamula mOl-R 0-50 ■ - ■ ■ ■ ■ ■Lima lima eMi-R 5-110 ■ — — — — —Limatula sulcata eMi-R 3-100 ■ ■ ■ ■ — — —Limatula ovata ePli 18-2000 ■ — — — — —Limatula subovata ePli-R - ■ — — — — — —Limaria tuberculata eMi-R 28-3500 ■ ■ ■ — — — —Limaria hians IMi-R ? ■ — — — — —Limaria loscombei mMi-R 0-100 — - ■ — — —Neopycnodonte cochlear ePli-R 35-100 ■ - - - - - -Ostrea edulis mMi-R 27-1500 - ■ ■ ■ ■ —Lucinoma (L.) b. borealis 101-R 0-90 ■ ■ ■ ■ ■ ■ —Parvilucina (Cavilinga) scaldensis e-lPli 0-500 ■ ■ ■ ■ ■ ■ -

208 Robert MARQUET

SPECIES RANGE BAT.K L O

AOC

0Ab

KS

M

Ctena (C.) decorata mPli _ _ ■ ■ ■ _ _ _Thyasira (T.) flexuosa mOl-R - ■ - - - - - -Leptaxinus (L.) ferruginosum ePli-R 10-2000 ■ - - - - - -Diplodonta (D.) rotundata lOl-R 20-1100 ■ ■ - - - - -Diplodonta (Zemysina) brocchii e-mPIi 0-100 ■ ■ ■ ■ ■ - -Felaniella (F.) astartea ePli-ePle - - ■ ■ ■ ■ -Scacchia (S.) elliptica mMi-R ? - ■ ■ - - -Semierycina (S.) kautskyi e-lPli - ■ ■ ■ ■ ■ ■ -Semierycina (S.) nitida ePli-R ? - ■ ■ ■ - - -Erycina (E.) depressa ePli - ■ ■ - - - - -Kellia suborbicularis eMi-R 0-120 ■ ■ - - - - -Aligena (A.) orbicularis ePli - ■ - - - - -Bornia deltoidea eMi-mPli - - ■ ■ - - -Montacuta substriata eMi-R 0-600 - ■ ■ ■ - - -Mioerycina (Phascoliophila) coarctata eMi-lPli - ■ ■ ■ ■ - ■ -Mysella (M.) bidentata eMi-R 0-2500 ■ ■ ■ ■ - - -Tellimya ferruginosa ePli-R ? ■ ■ - - - - -Laseina ambigua ePli-ePle - ■ ■ ■ ■ - - -Laseina cf. farnesiniana ePli-ePle - - ■ - ■ - - -Glans (Centrocardita) ampla IMi-ePli - ■ ■ - - - - -Glans (Centrocardita) scaldensis ePli - - ■ - - - - -Cyclocardia (C.) o. orbicularis ■ ■ - - - - -Cyclocardia (C.) o. chamaeformis eMi-ePle - - ■ ■ ■ - - -Cyclocardia (Scalaricardita) scalaris mMi-lPli - “ ■ ■ ■ ■ ■ -Pteromeris (Coripia) corbis mMi-R 7 - ■ - - - - -Glibertia prosperi e-mPli - - ■ ■ ■ - - -Erycinella pygmaea IMi-mPli - ■ ■ - - - - -Isocrassina basteroti e-lPli - ■ ■ ■ ■ ■ - -Isocrassina omalii e-mPli - ■ ■ - - - - -Isocrassina bipartita e-mPli - ■ - - - - -Isocrassina fusca ariejansseni IMi-ePli - ■ ■ - - - - -Isocrassina mutabilis ePli - ■ - - - - -Isocrassina altenai ?eMi-mPIi - ■ ■ - - - -Carinastarte trigonata IMi-ePli - ■ - - - - -Astarte (A.) incerta m-lPli - ■ ■ ■ ■ ■ -Astarte (A.) c. corbuloides ePli - ■ ■ - - - - -Astarte (A.) galeotti IMi-ePli - ■ - - - - -Astarte (Digitariopsis) o. obliquata - ■ ■ ■ ■ ■ -Astarte (Digitariopsis) o. burtinea IMi-IPli - ■ - - - - -Digitaria e. excurrens eMi-ePli - ■ ■ - - - - -Digitaria digitaria IMi-R 15-250 ■ ■ ■ ■ ■ ■ -Digitaria forbesi IMi-mPli - ■ ■ - ■ - - -Goodallia (G.) triangularis mMi-R ? ■ ■ ■ ■ - - -Goodallia (G.) parvula IMi-ePli - ■ - - - - -Goodallia (G.) pseudopygmaea mMi-ePIi - ■ - - - - -Parvicardium exiguum IMi-R 0-55 ■ - - - - -Plagiocardium scabrum ePli-R ? ■ ■ ■ ■ - - -Plagiocardium papillosum eMi-R 1-600 ■ - - — - -Nemocardium (Microcardium) doelensis ePli - - ■ - - - - -Laevicardium (L.) decorticatum e-lPli - ■ ■ ■ ■ ■ ■ -Laevicardium (Dinocardium) parkinsoni IPIi-ePle - - - - - - ■ -Cerastoderma edule hostiei mPli-R 0-10 - - ■ ■ ■ -Mactra (M.) glauca IMi-R 0-44 - - ■ - - ■ -Spisula (S.) arcuata e-lPli - - ■ ■ ■ ■ ■ -Spisula (S.) obtruncata e-lPli - ■ ■ ■ ■ ■ ■ -Spisula (S.) inaequilatera m-lPli - - - ■ - - -Spisula (S.) albertantonorum mPli - - - - ■ - -Lutraria (L.) scaldensis e-lPli - ■ ■ ■ ■ - ■ -Cultellus (C.) cultellatus eMi-lPli - ■ - ■ ■ ■ ■ -Ensis complanatus m-lPli - - - ■ ■ - ■ -Ensis hausmanni lOl-lPIi - ■ ■ ■ ■ ■ -Angulus (Moerella) donacinus eMi-R 0-200 - ■ ■ ■ ■ ■ ■Angulus (Peronaea) b. benedeni lOl-lPli - - - ■ ■ ■ ■ ■Arcopagia (A.) c. crassa mMi-R 0-155 — ■ ■ ■

Pliocene bivalves North Sea Basin 209

SPECIES RANGE BAT. K L 0A0C

0Ab

KS

M

Arcopagia (Arcopagiopsis) b. tenuilamellosa ePli-R 0-750 ■ ■ _ __ — _ _Gastrana laminosa mMi-Pli — ■ ■ ■ ■ ■ ■ —Macoma (M.) obliqua ePli-mPle - ■ ■ ■ ■ ■ ■ -Macoma (M.) praetenuis IPIi-lPle - - - - - ■Donax (Capsella) variegatus mPli-R 0-10 - - ■ - - -Gari (Psammobella) costulata mMi-R 0-460 - ■ - - - - -Gari (Psammobia) fervensis eMi-R 0-110 - ■ ■ ■ ■ ■ -Gari (Gobraeus) depressa ePli-R 0-50 - ■ ■ ■ - - -Abra (A.) prismatica IMi-R 0-40 ■ ■ ■ ■ - - -Abra (Syndosmya) a. alba mMi-R 0-60 - ■ ■ - ■ -Solecurtus scopulus mMi-R 0-110 - ■ ■ ■ - - -Arctica i. islandica eOl-R 0-500 ■ ■ ■ ■ ■ ■ ■Pygocardia rustica IMi-lPli - ■ ■ ■ ■ ■ ■ -Coralliophaga (C.) lithophagella mMi-R 30-200 ■ ■ - - - - -Glossus (G.) humanus mMi-R 7-250 ■ ■ - - - - -Venus (Dosina) casina IMi-R 5-200 - ■ ■ ■ ■ - -Venus (Ventricoloidea) pseudotumida ePli - ■ ■ - - - - -Gouldia (G.) minima eMi-R 0-200 ■ ■ - - - - -Pitar (P.) r. rudis eMi-R 0-80 ■ ■ ■ ■ - - -Callista (C.) chione eMi-R 0-180 - ■ - - - - -Dosinia (Asa) lupines lincta mMi-R 0-200 - . ■ ■ ■ ■ ■ -Dosinia (Pectunculus) cf. exoleta ePli-R 0-70 ■ - - - -Paphia rhomboides ePli-R 0-180 - ■ ■ ■ ■ ■ -Clausinella imbricata IMi-mPli - — ■ ■ ■ — -Timoclea (T.) ovata mMi-R 4-200 ■ ■ ■ ■ ■ ■ -Lajonkairea rupestris lupinoides 1-mPli - ■ ■ ■ ■ - -Mya (M.) truncata gudmunduri ePli-R 0-75 - - ■ ■ ■ ■ -Mya (Arenomya) arenaria lata IPIi-R 0-75 - - - - - ■ -Sphenia binghami IMi-R 1-60 - - - ■ - -Corbula (Varicorbula) gibba gibba lEo-R 0-250 ■ ■ ■ ■ ■ ■ ■Lentidium (L.) complanatum m-IPli - - - - ■ - ■ -Spheniopsis jugosa ePli - - ■ - - - -Gastrochoena (G.) dubia eMi-R 5-60 ■ — ■ - - -Hiatella (H.) arctica mOl-R 0-1400 ■ ■ ■ ■ ■ ■ ■Cyrtodaria angusta eMi-lPli - ■ ■ ■ ■ ■ ■ -Panomya trapezoides mMi-ePle - - - - ■ - ■ -Panopea (P.) glycimeris IMi-R 2-80 ■ ■ ■ ■ ■ ■ -Turneria cylindrica e-mPli - ■ - - ■ - -Turneria angulata e-mPli - ■ - - ■ - -Barnea (B.) cylindrica e-IPli - - - - - - ■ -Zirfaea crispata ePli-R 0-7 - - - ■ - ■ -Xylophaga sp. - ? - - - - - ■ -T eredinidae - ? ■ - - ■ - ■ -Pholadomya (P.) hesterna IMi-mPli - ■ ■ - ■ — — —Pandora (P.) pinna ePli-R 27-80 ■ ■ ■ ■ - - -Lyonsia (L.) mermuysi eMi-mPli - ■ ■ ■ ■ - - r-Cochlodesma (Bontaea) complanatum e-mPli - ■ ■ - ■ - - -Thracia (T.) altenai e-mPli - — ■ ■ ■ — — -Thracia (T.) i. inßata eMi-mPli - ■ - ■ - - - -Thracia (T.) convexa ePli-R 0-800 — — ■ - — — —Thracia (T.) detruncata e-mPli — ■ ■ - ■ — — . —Thracia (T.) cf. villosiuscula ePli-R ? - ■ - — — - -Poromya granulata IMi-R 30-600 ■ ■ - - - - -Cuspidaria (C.) rostrata ePli-R ? ■ - - - - - -Cuspidaria (C.) cuspidata eMi-R 20-250 ■ - - - - - -Cuspidaria (C.) cf. obesa ePli-R ? - ■ - - - - -Cardiomya costellata eMi-R 18-200 ■ - - - - - -Verticordia (V.) cardiiformis IMi-lPli - ■ ■ - - - - -

Total number o f species 110 117 82 90 44 53 8-9

Number of extinct species 54 57 38 45 21 29 2-3

% o f extinct species 49 49 46 50 48 55 34

210 Robert MARQUET

I I i i

45

N 40

m 35

30

25

20

15

10

5 0

S S i* i I

Fig. 2 — Bathymetric distribution o f the bivalve species in the Fig. 3 Kattendijk Fm. in the Kallo and Doei sections.

Bathymetric distribution o f the bivalve species in the Luchtbal Sand Mbr. in the Kallo and Doei sections.

m 30

lllr U L T -l

Depth (m)

Fig. 4 Bathymetric distribution of the bivalve species in the Oorderen Sand Mbr. in the Kallo and Doei sections.

liimuuT

Fig. 5

? ? 8 S £ S 8 8 8 8 8 8 8 8 3 8 8 8» 3 * 1 * 2 « ¡ I jj 1 § I I I I I !Depth (m)

Bathymetric distribution of the bivalve species in the Kruisschans Sand Mbr. in the Kallo and Doei sections.

gives shallower depth values: for the Kattendijk Fm. the only common bathymetry for all species is between 45 and 55 m, except for Bathyarca philippiana ( N y s t , 1848), which is found alive only at a depth of 150 m. A possible explanation for the latter fact is the lack of Recent records o f this rare species. Moreover in the Oorderen Sand Mbr. an anomaly was discovered: Donax variegatus (G m e lin i, 1791), which today occurs at a shallower depth than all other bivalve species in the same level. However, this is a very rare species in the sections examined and all specimens were eroded, indicating an allochthonous origin. With the second method, the importance of species with a limited bathymetric range is emphasized, whereas the importance o f taxa with an extremely wide range (e.g. from 0 to 2500 meters for Montacuta bidentata ( M o n t a g u , 1803)) is reduced. The resulting values are given in Tables 2 and 3.

Table 3 — Number o f bivalve species with known bathymetric range and depth o f deposition, according to the distribution o f these species, calculated as average value and by common overlap o f the species.

Stratigraphieunit

Number o f species

Medialdepth

Commonoverlap

Kattendijk Fm. 46 50-70 m 45-55 m

Luchtbal Sand Member

47 40-60 m 40-50 m

Oorderen Sand Member

39 40-50 m 35-45 m

Kruisschans Sand Member

27 20-50 m 15-55 m

Pliocene bivalves North Sea Basin 211

Discussion

G a e m e r s (1975a, b) discussed the depth of the different members found in the Kallo sections (excluding the Luchtbal Sand Mbr. not found at that time in the research area) based on the occurrence of brachiopods, molluscs, otoliths and sedimentary structures. G a e m e r s (1975a) estimated the depth of deposition of the Kattendijk Fm. at minimum 30 to 50 m, but pointed out that some o f the otoliths are characteristic of genera, now living deeper than 100 m. He concluded, as is stressed here also, that data on genus level should not be used indiscriminately for the assessment o f bathymetry. For the Oorderen Sand Mbr., G a e m e r s (1975b) estimated the depth at minimum 10 to 20 m, for the Kruisschans Sand Mbr. less than 10 m. For the two last mentioned deposits, this is shallower than our data indicate, whereas the results for the Kattendijk Fm. are comparable.

N u y t s (1990) investigated the biostratigraphy and lithostratigraphy based on benthic Foraminifera for the deposits at Kallo, except for the Luchtbal Sand Mbr. He concluded that his data were too variable to allow a palaeoecological conclusion and placed the Kattendijk Fm. at a depth of at least a few tens o f metres, not very close to the continent. The Oorderen and Kruisschans Sand Mbrs (between which he could not distinguish by means of the foraminiferal content) he classified as nearshore environments.

The data presented here for the bivalve fauna o f the Belgian part of the North Sea Basin cannot be easily compared with molluscan data from the British or Dutch coeval deposits. Indeed, first a systematic revision is needed to ascertain that the species under discussion are indeed different from each other and from Recent material. Then, as complete as possible distribution lists should be assembled per locality or member. However, the latest complete reviews from the British Pliocene bivalves are from W o o d (1851-1882), while the gastropods were treated subsequently by H a r m e r (1914-1925). Especially the last author split up his material excessively, so that the total number of species is highly overestimated and the number of extinct and endemic species is exaggerated. The Dutch material from borings is extensive, but only a small part has been published, the bivalves by H e e r i n g (1950). This author does not divide his samples according to different assemblages or members, so interpretation is difficult; a systematic revision of this material is needed.

Acknowledgem ents

I wish to thank the "M inisterie van de Vlaamse Gemeenschap, Afdeling Zeeschelde” and the staff members o f the TV Verrebroekdok and Deurganckdok for giving access to outcrops in Kallo and Doei. A. V. Dhondt gave me access to the collections o f the Royal Belgian Institute o f natural Sciences at my disposal and encouraged me to present this work as a poster at the "P . Bultynck Meeting” at the IRScNB in 2003.

References

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Robert M a r q u e tDepartment of PalaeontologyRoyal Belgian Institute of Natural SciencesRue Vautier 29B - 1000 BrusselsBelgium

Typescript submitted: May 10, 2004 Revised typescript received: July 24, 2004